1. Technical Field of the Invention
The present invention relates generally to an emergency call terminal device which may be employed in automotive vehicles to transmit, in the event of an emergency, data on the current location of the vehicle and registered information about the vehicle to an emergency center controlling an emergency call system to establish an emergency call connection to a police station, a fire department, etc. and more particularly to an improved structure of an emergency call terminal device designed to detect a failure of the device such as a disconnection of an emergency call button from a control circuit.
2. Background Art
There are known automotive emergency call terminal devices equipped with an emergency telephone call unit which establishes a call connection to an emergency center through a base station of a typical telecommunication network. The automotive emergency call terminal devices are generally broken down into two types: the first type in which an emergency call button is manually pushed to output a calling signal and the second type in which a calling signal is outputted automatically in response to an output from an airbag sensor or a collision sensor. Some of the automotive emergency call terminal devices of the first type are designed to detect an accidental disconnection of a signal line from the emergency call button, resulting in formation of an open circuit. Specifically, the terminal devices of this type include a control circuit which monitors the depression of the emergency call button and disconnection of the signal line leading to the emergency call button to perform necessary operations. The connection of the emergency call button and the control circuit is established through a switch upon depression of the emergency call button. The switch connects at one end with the emergency call button and at the other end with ground. The control circuit receives an input signal of a ground potential when the emergency call button is pushed, an input signal of a voltage depending upon a resistance ratio of a pull-up resistor to the switch when the emergency call button is not pushed, and an input signal of a pull-up voltage when the signal line is disconnected undesirably from the emergency call button, resulting in formation of an open circuit of the control circuit. This prior art structure, therefore, encounters a drawback in that when the signal line is disconnected from the emergency call button, thereby causing the control circuit to be short-circuited to ground, it will cause the ground potential to appear at the control circuit, thereby making it difficult to discriminate between the disconnection of the signal line leading to formation of the open circuit of the control circuit and one leading to formation of the short circuit of the control circuit to ground.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide an emergency call terminal device capable of discriminating between an open circuit-caused failure resulting from formation of an open circuit of an internal circuit of the device caused by an accidental disconnection of the internal circuit from an emergency call push button and a short circuit-caused failure resulting from formation of a short circuit of the internal circuit to ground caused by an accidental disconnection of the internal circuit from the emergency call push button.
According to one aspect of the invention, there is provided an emergency call terminal device which comprises: (a) an emergency call circuit responsive to a call request signal to output an emergency call signal to a preselected emergency center; (b) an emergency call switch connected to the emergency call circuit, the emergency call switch being designed to be switched between an ON-state and an OFF-state, when the emergency call switch is brought into the ON-state, the call request signal being provided to the emergency call circuit; and (c) an operating state signal producing circuit producing a first, a second, a third, and a fourth operating state signal different in voltage level from each other for enabling the emergency call circuit to discriminate the ON-state, the OFF-state, an open circuit-caused failure, and a short circuit-caused failure. The first operating state signal is produced when the emergency call switch is in the ON-state. The second operating state signal is produced when the emergency call switch is in the OFF-state. The third operating state signal is produced when the open circuit-caused switching failure results from formation of an open circuit of the emergency call circuit caused by a disconnection of the emergency call circuit from the emergency call switch. The fourth operating state signal is produced when the short circuit-caused failure results from formation of a short circuit of the emergency call circuit caused by a disconnection of the emergency call circuit from the emergency call switch.
In the preferred mode of the invention, the operating state signal producing circuit includes a first resistor connected to a first and a second end of the emergency call switch, a second resistor connected between the first end of the emergency call switch and ground, and a third resistor connected between a voltage source and a junction of the emergency call circuit and the second end of the emergency call switch.
The operating state signal producing circuit may alternatively include a first resistor connected between a first end of the emergency call switch and ground, a second resistor connected between a second end of the emergency call switch and ground, and a third resistor connected between a voltage source and a junction of the emergency call circuit and the second end of the emergency call switch.
The operating state signal producing circuit may alternatively include a first and a second resistor connected in series between a first end of the emergency call switch and ground, a third resistor connected between a junction of the first and second resistors and the second end of the emergency call switch, and a fourth resistor connected between a voltage source and a junction of the emergency call circuit and the second end of the emergency call switch.
The operating state signal producing circuit may alternatively include a first and a second resistor connected in series between a first end of the emergency call switch and ground, a third resistor connected between ground and a second end of the emergency call switch, and a fourth resistor connected between a voltage source and a junction of the emergency call circuit and the second end of the emergency call switch.
The operating state signal producing circuit may alternatively include a first resistor connected to a first end of the emergency call switch leading to ground and a second end of the emergency call switch, a second resistor connected between the second end of the emergency call switch and the emergency call circuit, and a third resistor connected between a voltage source and a junction of the emergency call circuit and the second end of the emergency call switch.
The operating state signal producing circuit may alternatively include a first resistor connected to a first end of the emergency call switch and a first junction leading to the emergency call circuit, a second resistor connected to the first junction and ground in parallel to the emergency call switch and the first resistor, and a third resistor connected between a voltage source and a second junction which connects the emergency call circuit and the first junction.
The operating state signal producing circuit may alternatively include a first resistor connected between a first end of the emergency call switch and ground, a second resistor connected between a second end of the emergency call switch and the emergency call circuit, a third resistor connected between the second end of the emergency call switch and ground in parallel to the emergency call switch and the first resistor, and a fourth resistor connected between a voltage source and a junction of the emergency call circuit and the second resistor.
The operating state signal producing circuit may alternatively include a first resistor connected between a first end of the emergency call switch and ground, a second resistor connected between a second end of the emergency call switch and a first junction leading to the emergency call circuit, a third resistor connected between the first junction and ground in parallel to the emergency call switch and the first and second resistors, and a fourth resistor connected between a voltage source and a second junction which connects the first junction and the emergency call circuit.
The operating signal producing circuit produces the third and fourth operating state signals using functions of pull-up and pull-down resistors.
The emergency call circuit receives the first, the second, the third, and the fourth operating state signals through one of an A/D converter and an A/D converter built in a microcomputer.
According to another aspect of the invention, there is provided an emergency call system which comprises an emergency center and an emergency call terminal device. The emergency call terminal device includes, (a) an emergency call circuit responsive to a call request signal to output an emergency call signal to the emergency center, (b) an emergency call switch connected to the emergency call circuit, the emergency call switch being designed to be switched between an ON-state and an OFF-state, when the emergency call switch is brought into the ON-state, the call request signal being provided to the emergency call circuit, and (c) an operating state signal producing circuit producing a first, a second, a third, and a fourth operating state signal different in voltage level from each other for enabling the emergency call circuit to discriminate the ON-state, the OFF-state, an open circuit-caused failure, and a short circuit-caused failure. The first operating state signal is produced when the emergency call switch is in the ON-state. The second operating state signal is produced when the emergency call switch is in the OFF-state. The third operating state signal is produced when the open circuit-caused switching failure results from formation of an open circuit of the emergency call circuit caused by a disconnection of the emergency call circuit from the emergency call switch. The fourth operating state signal is produced when the short circuit-caused failure results from formation of a short circuit of the emergency call circuit caused by a disconnection of the emergency call circuit from the emergency call switch.